The modern mobile telecommunications network offer a new attractive feature: roaming between several operationally independent public land mobile networks (PLMN). Roaming between PLMNs in different countries is called international roaming. Normally a mobile subscriber has a subscription relationship with a single PLMN. This specific PLMN is called the home PLMN of the mobile subscriber. Service can be obtained also from other PLMNs, depending among other conditions on subscriptions. For example a mobile subscriber who has a subscription relationship with a national PLMN in Finland may also make outgoing calls and receive incoming calls through another PLMN for example in Germany, if there are agreements between the operators of the two PLMNs. The other PLMN is sometimes called visited PLMN. One example of a standardized system supporting international roaming is the pan-European digital mobile radio system GSM (Global System for Mobile Communications) and its modification DCS1800 (Digital Communications System). The GSM supports both mobile station roaming in which a mobile station moves from one PLMN to another, and a subscriber identification module (SIM) roaming in which only a SIM moves. The latter may allow also roaming between PLMNs having incompatible radio interfaces, such as different operating frequencies.
A significant disadvantage that makes the international roaming less attractive is the cost of the international calls. Typically a calling party is charged for the first half of the routing to the home PLMN of the called mobile subscriber, and the called party is charged for the second half of the routing from the home PLMN to the visited PLMN. Thus, the called party has to pay the expensive international call charges also for calls which he/she does not want to receive at all. The expensive call charges will discourage the mobile subscriber from extensively using the roaming service. Actually, it is believed that most of the mobile subscribers will have incoming call barried while roaming.
In the international calls originating from the roaming mobile station, the routing of the call is always optimized. However, routing of the call destinating to a roaming mobile station is not optimized, and consequently, double international legs and very high international call charges may unnecessarily arise. This problem will be explained by means of an example with reference to FIGS. 1 and 2.
In FIG. 1 a PSTN subscriber in Australia makes an international call to a mobile station MS having a home PLMN in Germany and being currently located in the home PLMN. The PSTN subscriber dials a prefix 0011 and the mobile station ISDN number MSISDN including a country code CC, a national destination code NDC and a subscriber number SN. The prefix 0011 is an international call code in Australia, and consequently, the local PSTN exchanges route the call to the international PSTN gateway in Australia. The international PSTN gateway routes the call to a destination country, i.e. Germany, according to the country code CC. The international PSTN gateway in Germany will route the call to the gateway MSC in the home PLMN of the called MS according to the national destination code NDC. The gateway MSC interrogates the home location register HLR of the called MS according to the NDC SN and then route the call to the called MS according to the mobile station roaming number MSRN obtained via the HLR. In the above case the PSTN subscriber will pay charge for the whole call.
In FIG. 2 it is assumed that the called MS is roaming in a visited PLMN in Australia. Similar to the above case in the first half of the routing, the call is first of all routed to the home country and the home PLMN of the called MS. The mobile station roaming number MSRN is then obtained via the HLR and the call will then be routed to the MS in Australia via another international leg. As a consequence, two international legs and the associated call charges are involved in the call. The charge for the first international leg will be paid by the calling party and the charge for the second international leg will be paid by the called MS.
There are several reasons for such a strange, costly routing configuration in the international calls. The current routing topology of telecommunication network has hundreds of local exchanges, each exchange having a routing table to find the right route for the call. The maintenance of these routing tables is very time consuming and complicated, and therefore, the tables are always made as simple as possible. Thus, only a few digits of the dialed telephone number are analysed at each section of the call route. For example, when a PSTN subscriber dials a telephone number 0011 CC NDC SN, the local PSTN exchange will only recognise the international prefix 0011 and then go straight to the international gateway, as in the cases shown in FIGS. 1 and 2. A local PSTN exchange ignores the content of the following digits in the telephone number. The GSM specifications has suggested that any equipment along the call handling path can be an intelligent gateway that performs the HLR interrogation and thereby optimises the routing of the call. However, the implementation of this function would require implementation of GSM signalling capability into the existing international gateway PSTN exchanges which will require some very costly software development or maybe even impossible because of the old switching platforms. The current specifications of CCITT could actually overcome the problem by using the rerouting capability of the international gateway relating to the CCITT Common Channel Signalling System 7 (C7). However, international carrier and PTT are reluctant to implement the automatic re-routing function into their international gateways because:
The international circuits do not always support the use of common channel signalling system 7. The re-routing function will not be easily implemented in such a non-C7 systems. PA1 The normal international telephony does not require that kind of function. Only the automatic international roaming of GSM or DCS1800 or the international call forwarding cases require the re-routing function. This is a relatively small percentage of the total traffic. PA1 The high development costs of the re-routing function would only lead to a lower revenue to them. Since optimised routing would result in no double international legs and thereby in less international toll charges. PA1 a first subscriber of a first telecommunications network in a first country dials a predetermined prefix reserved only for international mobile subscriber terminating calls in the first country, and a mobile station ISDN number of a called mobile subscriber having a home PLMN in a second country, PA1 the first telecommunications network in the first country routes the call according to said prefix to a predetermined network element in the first country having an international PLMN interrogation capability, PA1 the predetermined network element makes an interrogation to the home PLMN of the called mobile subscriber in the second country, PA1 the home PLMN of the called mobile subscriber sends to the predetermined network element a mobile subscriber roaming number allocated for the called mobile subscriber, PA1 the predetermined network element routes the call directly to a destination according to the mobile subscriber roaming number.
Therefore there is a need for improved international routing in order to avoid unnecessary international legs of PSTN originating and mobile terminating calls in international automatic roaming.